This is a proposal for support for a RES Research Conference on Antigen Presenting Cells. There are a variety of cell types capable of presenting antigens to T lymphocytes and/or B lymphocytes. Recognition of antigen by specific lymphocytes is clearly of primary importance in the initiation of immune responses. It is known that T lymphocytes only recognize antigen when it is presented in association with molecules coded for by genes in the major histocompatability complex (MHC). Most T cells requires that an antigen presenting cell (APC) "process" the antigen and express in it association with the class II antigens of the MHC on the surface of the APC. For years macrophages have been considered the essential APC. More recently other cells including Langerhans cells, dendritic cells, and a number of other cells that express class II antigens, or can be induced to express class II antigens, have been included among important APCs. Most recently it has been documented that B cells can present antigen and that their specific immunoglobulin receptors can bind and concentrate the antigen. In addition, new information is now available regarding the processing events, the association of antigen with class II, and the molecular nature of the cellular interaction between the T lymphocyte and the APC. These recent developments raise important questions about the diversity, differentiation and regulation of APCs and this prompts us to propose this research conference. No other conference has focused on these issues and this conference should help disseminate the new information and focus future research. The development and exchange of ideas, perspectives, and techniques between scientists is the goal and ultimate justification for this conference. Developments in this area may offer an opportunity for immunological intervention in a wide variety of diseases including neoplastic diseases, allergic diseases, and infectious diseases.